A transmission equipped in a motorcycle typically includes a shift drum configured to rotate in association with an operation of a change pedal, and is configured to rotate the shift drum to cause a shift fork to pivot, thereby changing a gear position, namely, a gear mesh. Such a transmission is disclosed in, for example, Japanese Laid-Open Patent Application Publication Nos. Hei. 11-59217 and 2002-206628, and Japanese Utility Model Application Publication No. Hei. 2-107867. The transmission is equipped with a resistive contact sensor to detect a rotational angle of the shift drum corresponding to the gear position.
The resistive contact sensor typically includes an arm extending in a direction substantially perpendicular to a rotational axis of the shift drum, and a resistive element configured to contact a tip end of the arm. The resistive element is fixed to a vehicle body of the motorcycle, whereas the arm is rotatable integrally with the shift drum. According to the rotation of the shift drum, a contact point between the arm and the resistive element changes. The resistive contact sensor outputs a signal with a potential according to the contact point.
In motorcycles, an ECU (electric control unit) detects a rotational angle of the shift drum based on a signal output from the resistive contact sensor, and determines whether or not the gear position has been upshifted according to the rotation of the shift drum. If it is determined that the position has been upshifted, then the ECU causes an ignition device of an engine to reduce the number of times of ignition or to retard an ignition timing so that the engine can conform to a condition of the upshifted gear position.
In the conventional configuration, to detect the position of the shift drum after rotation by using the resistive contact sensor, a correct contact state between the resistive element and the arm in a rotational direction of the shift drum and in a direction perpendicular to the rotational direction cannot be maintained, causing the signal output from the sensor to include noise. As a result, a relatively long time is required to stabilize the value of the signal potential according to the stop position of the shift drum. In order to enable the engine to conform to the condition of the upshifted gear position by reducing the number of times of ignition or by retarding the ignition timing, it is necessary to determine whether or not the gear position has been upshifted, immediately after start of the rotation of the shift drum. Typically, the gear position is upshifted when an engine speed is increasing, i.e., during a cycle in which the ignition in the ignition device is short. For this reason, correct reduction of the number of times of ignition, etc, is unable to be accomplished unless it is determined whether or not the gear position has been upshifted, immediately after the start of the rotation of the shift drum.
In order to solve the above mentioned problem, in addition to the resistive contact sensor equipped in the transmission, a load cell is mounted to a shift rod, and based on a signal issued from the load cell, it is determined whether a rider has operated a shift pedal to upshift or downshift a gear position. In general, since the signal from the load cell has a small amplitude, an amplifier is disposed between the load cell and the ECU to amplify the signal. When the resistive contact sensor, the load cell, the amplifier, etc., are incorporated into the transmission, the transmission becomes expensive. In addition, the components to be maintained increase in number.